The Effects of Chlorine and Selenium in Cadmium Telluride Solar Cells By Thomas Fiducia Abstract Solar photovoltaics (PV) holds great promise to change the way that electricity is produced and used globally. As it stands, electricity is generated mainly by large coal and gas-fired power stations, which are expensive to build and rely on a fuel supply that becomes more expensive over time. By contrast, the costs of solar PV are falling rapidly, and solar is already producing electricity at lower levelised costs than coal and gas power stations. Moreover, it can do so with a very low environmental impact, and since it is a ‘distributed’ power source that does not require a fuel supply, also improves access to electricity and the overall security of supply. However, if these benefits are to be realised, deployment of solar PV needs to continue to scale significantly. Solar PV currently supplies only ~3% of worldwide electricity demand, and demand for electricity is set to nearly double by 2050 as a result of the electrification of heating and transport and rising living standards. In order to continue its rapid growth and help to meet a significant portion of future electricity demand, solar module efficiencies need to continue to rise and production costs need to decrease further. Fast-deposited thin-film PV technologies like cadmium telluride (CdTe) offer a promising route to achieve the necessary price decreases and industry scale-up because they are intrinsically less expensive to produce than the incumbent silicon PV modules, which require careful crystal growth and individual processing of each wafer, cell and module. The downside of fast thin-film deposition however is that the devices invariably have polycrystalline absorber layers with small crystal ‘grains’, and high defect densities. This not only limits power conversion efficiency compared to single crystal counterparts like silicon, but also makes the devices much more microstructurally and compositionally complicated, and hence more difficult to characterise and control. In particular, device-level characterisation techniques that were developed for homogeneous single crystal 1 absorber layers are not sufficient to resolve the complexities of thin-film cells and high-resolution characterisation techniques have been under-used, slowing device development. Here we use high-resolution correlative characterisation techniques to investigate the effects of two elements that are vital to producing high efficiency cadmium telluride solar cells – chlorine and selenium. Using 3-dimensional NanoSIMS compositional mapping we find that following the essential cadmium chloride heat treatment, chlorine is not just present in grain boundaries – where it is known to have a passivation effect – but permeates every region of the CdTe absorber layer. It is found segregated at the front interface between the CdTe and the buffer layer, at incoherent twin boundaries that span grain interiors, and at dopant concentrations in grain interiors. In selenium- alloyed CdTe devices we use high resolution NanoSIMS and SEM-based cathodoluminescence, on the same area of the absorber, to reveal that selenium alloying lowers non-radiative recombination levels in CdSeTe grain interiors, helping to explain the record performance of selenium-graded devices. We then use TEM-based cathodoluminescence to show that selenium also has a passivation effect on grain boundaries in CdSeTe, which is the first time that high-resolution TEM-CL mapping has been achieved on a solar cell. Together, these results help to explain how cadmium telluride devices have achieved efficiencies of over 22%, despite their fast absorber layer deposition and small grain sizes. The results suggest new routes for further efficiency improvement of CdTe solar cells, including by increasing selenium concentrations at grain boundaries and in the bulk material at the back of the absorber layer. This can reduce costs further for what is currently the lowest cost of all solar and fossil fuel electricity generation technologies, and hence help to spread the cost, security, and environmental benefits of solar photovoltaics. It is also intended that the work will encourage more high resolution, correlative characterisation of thin-film PV technologies in general. 2 Publications List - T. A. M. Fiducia, B. G. Mendis, K. Li, C. R. M. Grovenor, A. H. Munshi, K. Barth, W. S. Sampath, L. D. Wright, A. Abbas, J. W. Bowers and J. M. Walls, ‘Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells’, Nature Energy, volume 4, pages 504–511 (2019). - T. A. M. Fiducia, A. Howkins, A. Abbas, B. Mendis, A. H. Munshi, K. Barth, W. S. Sampath, and J. M. Walls, ‘Passivation of Grain Boundaries in Selenium-alloyed CdTe Solar Cells’, manuscript in preparation. - T. A. M. Fiducia, K. Li, A. H. Munshi, K. Barth, W. S. Sampath, C. R. M. Grovenor, and J. M. Walls, ‘3D Imaging of Selenium and Chlorine Distributions in Highly Efficient Selenium-Graded Cadmium Telluride Solar Cells’, IEEE Journal of Photovoltaics, vol. 10, no. 2, pp. 685–689, Mar. 2020. - M. J. Watts, T. A. M. Fiducia, B. Sanyal, R. Smith, J. M. Walls, and P. Goddard, “Enhancement of photovoltaic efficiency in CdSexTe1− x (where 0 ⩽ x ⩽ 1): insights from density functional theory,” J. Phys. Condens. Matter, vol. 32, no. 12, p. 125702, Mar. 2020. - T. A. M. Fiducia, K. Li, A. H. Munshi, K. Barth, W. S. Sampath, C. R. M. Grovenor, J. M. Walls, ‘3D Distributions of Chlorine and Sulphur Impurities in a Thin-Film Cadmium Telluride Solar Cell’, MRS Advances, vol. 3, no. 56, pp. 3287–3292, May 2018. - T. A. M. Fiducia, A. H. Munshi, K. Barth, D. Proprentner, G. West, W. S. Sampath, and J. M. Walls, ‘Defect Tolerance in as-deposited Selenium-alloyed Cadmium Telluride Solar Cells’, IEEE 2018 World Conference on Photovoltaic Energy Conversion. - T. A. M. Fiducia, K. Li, A. H. Munshi, K. Barth, W. S. Sampath, C. R. M. Grovenor, and J. M. Walls. ‘Large Area 3D Elemental Mapping of a MgZnO/CdTe Solar Cell with Correlative EBSD Measurements’, IEEE 2018 World Conference on Photovoltaic Energy Conversion. - T. A. M. Fiducia, A. Abbas, K. Barth, W. S. Sampath, J. M. Walls, ‘Intragranular Defects in As- Deposited and Cadmium Chloride-Treated Polycrystalline Cadmium Telluride Solar Cells’, 43rd IEEE Photovoltaic Specialist Conference (PVSC), 2016 3 Acknowledgements There are several people that I would like to thank for their help and support during the PhD. First, I would like to thank my supervisor, Professor Mike Walls, who gave me the opportunity to pursue the PhD. Mike is perhaps the most supportive (but not everbearing) supervisor you could ask for. I have enjoyed the research process hugely and that is largely down to him and his enthusiasm and positive attitude. I would also like to thank our many collaborators and co-authors, who have been instrumental in making the work for this thesis possible. At Colorado State University, many people including Amit Munshi, Tushar Shimpi, Kurt Barth, Sampath, and Jim Sites have provided general support as well as their world class CdTe samples. They were also excellent hosts during my three months in Fort Collins. At Oxford, Chris Grovenor, Kexue Li and Junliang Liu performed the vital NanoSIMS measurements. I would particularly like to thank Kexue (now at Manchester) for his long hours gathering data at the Cameca. Budhika Mendis at Durham performed the SEM-based cathodoluminescence for chapter 5 and helped to make sense of the resulting data. He also put in a lot of time helping with publication of papers generally, which I am extremely grateful for. Ashley Howkins at Brunel performed the TEM-CL measurements for Chapter 7. At Loughborough there is a great network of people within the offices and labs at CREST that help each other out and I am particularly thankful to them. Jake bowers has been an excellent second supervisor and is always ready to help with good advice and PV knowledge. Likewise, Patrick Isherwood is a fountain of useful information and I have appreciated our solar-based chats. Ali Abbas is a fantastic microscopist and I am grateful for all his help over the years, and for our regular talks and meetings with Mike. At the LMCC, Ryan MacLachlan, Zhaoxia Zhao, and many others have always been on hand to help. My fellow CdTe PhD students and desk mates Rachael Greenhalgh and Christos Potamialis have provided much amusement and support throughout the project. Staff and students on the CDT-PV (Centre for Doctoral Training in New and Sustainable Photovoltaics) were helpful and entertaining throughout the PhD, particularly during the rotational training program in first year. Lewis Wright was there from the beginning: at the CDT-PV training, in the office, and as my housemate throughout most of the last 4 years. Lewis was instrumental for the python-based plotting in Chapter 5, and has greatly enhanced my time so far at Loughborough. Likewise, Rhys Comissiong has provided a lot of helpful advice and insight as a PV ‘outsider’. Finally, I want to thank my parents; for putting up with me during lockdown, for taking extra interest in my work, and for supporting me all the way up to this point. I hope that this can be a small repayment for everything that you have done. 4 Table of Contents Abstract 1 Publications List........................................................................................................................................... 3 Acknowledgements ...................................................................................................................................