This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and the Government of Catalonia's Agency for Business Competitiveness (ACCIÓ). This research has also been supported by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the Spanish Ministry of Science, Innovation and Universities under the IGNITE project (ENE2017-87671-C3-1-R), and by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). Authors from IREC and the University of Barcelona belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the "Generalitat de Catalunya" (Ref.2017 SGR 862). M.P. thanks the Government of Spain for the Ramon y Cajal Fellowship (RYC-2017-23758). This research was also supported by Consejo Nacional de Ciencia y Tecnología (CONACYT) with the scholarship No. 433146 and with the project CB-CONACYT-255062. Finally, this research was also supported by Programa de Fortalecimiento de la Calidad Educativa (PFCE 2019) and by Project VIEP 2019.

We report the effect of an ultra-thin Al2O3 layer (down to 3 nm) as interface passivation strategy for the improvement of the performance of Cu2ZnSnS4/CdS based solar cells. After an initial optimization, the Al2O3 deposited by thermal evaporation is proved to improve the properties of the p-n junction. The fabricated devices showed an increment in Voc depending on the composition of the absorber, and an improvement in fill factor (FF) apparently related to the insulation of possible shunt-paths. Also, the impact on other optoelectronic parameters is discussed.