GROWTH and PROPERTIES of BULK Cztsse and Sb2se3 for SOLAR CELLS
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GROWTH AND PROPERTIES OF BULK CZTSSe AND Sb2Se3 FOR SOLAR CELLS Theodore Douglas Christopher Hobson Thesis submitted in accordance with the requirements of the UNIVERSITY OF LIVERPOOL For the degree of DOCTOR IN PHILOSOPHY Stephenson Institute for Renewable Energy Department of Physics May 2020 For Holly Abstract This thesis presents investigations into growth techniques to produce bulk polycrystalline and single crystals of the materials Cu2ZnSnS4xSe4(1-x) (CZTSSe) and Sb2Se3, in order to better understand bulk properties of relevance for photovoltaics. For CZTSSe, a technique to grow ~100 μm monograins from a molten NaCl/KCl mix proved successful for compositions 0 ≤ x ≤ 1 in increments of Δx = 0.1, with a more ordered kesterite phase demonstrated compared to directly synthesised feedstock. From this series, Vegard relations were measured as: a (Å) = -0.268(3)x + 5.6949(17) and c (Å) = - 0.516(6)x + 11.345(3) while Raman modes were observed to follow two-mode behaviour as: ωCZTSe -1 -1 (cm ) = (44.6 ± 1.6)x + (194.6 ± 0.8) and ωCZTS (cm ) = (7.1 ± 1.3)x + (329.0 ± 0.8). These relationships are intended to be used as reference data to aid the measurement of sulphur-selenium ratios in CZTSSe thin films. Samples consistent with single crystals of Sb2Se3 with 4 mm diameter and 1 cm length were grown via vertical Bridgman, with the {100} crystal planes evidenced through optical means and XRD. These crystals exhibited low-angle boundary defects consistent with ripplocations. They also demonstrated anisotropies in the polarised Raman spectra, with sample rotation able to resolve the overlapping 187 and 190 cm-1 peaks. The 253 cm-1 peak in the Raman spectra was consistent with reports of Sb2O3. Se-poor and stoichiometric Sb2Se3 single crystals were non-conductive, while Se-rich were p-type, and Sn and Cl were found to induce p-type and n-type doping respectively, with a maximum hole density from Sn-doping of 1015 cm-3 and electron density 17 -3 from Cl of 10 cm . Barrier heights of 0.37 eV for p-Sb2Se3 / In interfaces and 0.56 eV for n-Sb2Se3 / Au interfaces were estimated. Deep level electron traps at 400, 700 and 850 meV were detected in n- type single crystals, thought to result from VSe and VSe defects, while photoluminescence peaks at 0.9, 1.1 and 1.2 eV were assigned to donor-acceptor pair transitions resulting from Sn and Cl impurities. Thermally evaporated thin film Sb2Se3 / CdS solar cells were prepared from n-type, intrinsic and p- type doped source material, all reaching ~1 % efficiency. Of these, the p-type devices decreased in efficiency after ageing in air, while the intrinsic and n-type increased slightly, possibly due to Sb2O3 formation from oxidisation. i Declarations All of the measurements presented in this thesis were carried out by the author apart from the following, separated by institution: Stephenson Institute for Renewable Energy, University of Liverpool: 1. Oliver S Hutter: a. Collaborated with the author on the synthesis of CZTSSe feedstock and growth of crystallised solid solutions series (Sections 4.2.1, 4.2.3, 5.2.1, 5.2.2 and 5.2.3) b. Carried out differential scanning calorimetry of NaCl/KCl mix (fig. 4.1) and CZTSSe feedstock series (fig. 5.2) c. Prepared Sb2Se3 thin films by CSS (Section 4.3.5) and air-annealed them (Section 6.6.5.1) d. Carried out SEM for several samples (figs. 5.1, 6.4 and 7.19) 2. Jonathan D Major: a. Sputtered CdS layer and deposited thermally evaporated Sb2Se3 layers for thin film devices (Sections 4.3.7 and 7.7) b. Carried out deep-level transient spectroscopy measurements (fig. 7.12) 3. Nicole Fleck carried out Raman measurements of the crystallised CZTSSe solid solution series (0 ≤ x ≤ 1) (figs. 5.11 and 5.12a) Other departments at the University of Liverpool: 1. EBSD by Elisabetta Mariani in the Dept. of Earth, Ocean and Ecological Sciences (figs. 6.16a-c) 2. ICP-OES by Stephen Moss in analytical services at the Dept. of Chemistry (Table 5.2, figs. 5.3 and 5.4, Table 6.5) Other institutions: 1. ICP-MS by Chris Ottley and Emily Unsworth in the Department of Earth Sciences at the University of Durham (Table 6.6) 2. Quadrupole SIMS by Guillaume Zoppi in the Dept. of Mathematics, Physics and Electrical Engineering at the University of Northumbria (fig. 6.19) 3. Hall effect by Bhaskar Das in the Chemical Engineering and Materials Science Department at the University of Minnesota (fig. 7.3, Table 7.3) 4. Photoluminescence by Maarja Grossberg, Tavi Raadik and Juri Krustok at the Department of Materials Science at the Tallinn University of Technology (figs. 7.14a and 7.15) ii Acknowledgements I have had the privilege of working with colleagues who are both great scientists and great friends. I have learnt so much from the people around me that I can’t possibly relate all the ways in which they have made this work possible. I’ll simply start by saying it has been a pleasure to collaborate with Oliver Hutter, Nicole Fleck, Huw Shiel, Laurie Philips, Leanne Jones, Tom Featherstone, Matt Smiles, Jack Swallow, Sylvia Mariotti, Isabel Vazquez-Fernandez, Luke Daniels and Holly Edwards to produce work I feel we should all be very proud of. The same goes for my many other collaborators both inside and outside the University of Liverpool. You have all been a joy to work with. However, the work that makes up this thesis would also not have been possible without the invaluable help and assistance of Phil Murgatroyd, Tom Shalvey, Jessica Stoner, Tom Baines, Max Birkett, Annette Pressman, Joe Horne, Peter Yates, Manuel Apollo and Luke Thomas. There’s no way I’ve repaid even half of the help you’ve given me over the years. It’s not just direct collaboration that makes a PhD, and all of my colleagues have been brilliant people to spend time with both inside and outside the lab, making the Stephenson Institute an exciting and friendly place to work. I want to offer additional thanks to Leanne and Nicole for being wonderful desk-mates and making our end of the office the ‘cool corner’, at least in my mind. Having Tom F around to chat was another thing that made be glad to be where I was, and along with Jack Beane, provided a welcome excuse for me to wander around the office. Thanks to Olivia Voyce for always encouraging us to be better people and to Daniel Bromley and Alex Wright for standing up for the rights of students. Thanks to Joe for injecting humour into our lunchtimes and being an unbeatable fountain of trivia, and thanks to Phil for always being there to help me out, whatever the issue. Thanks to Holly for being a great CZTS bestie and to Hatem Amli for brightening up the office. Thanks to Oliver for teaching me a little of what he knows and most of what I know, even if not all the facts were what I’d consider ‘fun’. Thanks to Laurie for being an essential beacon of competence while the rest of us are bumbling around. Thanks to Tom B for introducing me to some iii great music, and to Matt for picking up the dad-joke baton after Tom went over to the dark side. Thanks also to Luke for leading the SIRE football team to victory (the first time) and to Jacob for organising games under lockdown that helped keep me sane. Thanks to Huw for reminding us that there’s more to life than work, Jack S for always having a great sense of humour and Tom S for putting up with all of us. Thanks to Pete, Sylvia and Annette for making me feel welcome when I first started. I also want to thank Bruna Baggio, Angeline Kasina, Kieran Routledge, Mark Forster, Gaia Neri, Stefano Mensa, Charlotte Smith, Liam Banerji, Daniel Cheung and Khezar Saeed for all helping to make the Institute the fun and social place it is. It goes without saying that I need to thank my supervisors Ken Durose and Jon Major for their support and encouragement as much as for their sympathy and patience with me. Thanks are always due to Vince Vasey for keeping the whole lab ticking over and I owe a great thanks to Tim Veal, Dmitry Schkukin, Jon Alaria, Frank Jaeckel, Vin Dhanak and Laurence Hardwick for all their help and for trusting me with their equipment. I want to thank my examiners Liam O’Brien and David Lane for their helpful discussions and feedback. Thanks also need to go to my parents Kate Hobson and Hugh Pearman, my brother Ben, sisters Harriet and Lydia and all my friends in other places. To my oldest friends Joe Drane, Ayla Richardson, Roman Sheppard-Dawson and Sophie Wolfson, I owe a great thanks. I want to thank Holly Gilson for being my wonderful and constant partner in all things. This work was finished during a difficult time for many people, and all I can do is say thank you to all my friends and family, and to wish you all the very best. This work, like me, is better for me having known all of you. iv Contents Abstract ................................................................................................................................................... i Declarations ........................................................................................................................................... ii Acknowledgements .............................................................................................................................. iii 1. Introduction ....................................................................................................................................... 1 1.1 Context for Photovoltaics.............................................................................................................. 1 1.2 Motivation for Researching CZTSSe and Sb2Se3 ......................................................................... 3 1.3 Structure of this Thesis ................................................................................................................. 9 1.4 References ................................................................................................................................... 10 2. Literature Review of CZTSSe and Sb2Se3 in Photovoltaics .......................................................