Rear surface passivation of ultra-thin CIGS solar cells using atomic layer deposited HfOx Gizem Birant, Jorge Mafalda, Romain Scaffidi, Jessica de Wild, Dilara Gokcen Buldu, Thierry Kohl, Guy Brammertz, Marc Meuris, Jef Poortmans and Bart Vermang EPJ Photovolt., 11 (2020) 10 Published online: 08 December 2020 DOI: 10.1051/epjpv/2020007