“…An assortment of ETL materials, including low work function metals or related salts, e.g., Ca ( Anagnostou et al, 2019 ; Ghosekar and Patil, 2019 ; Lian et al, 2019 ; Song et al, 2019 ; Yu et al, 2019 ) and LiF ( Zhao and Alford, 2018 ; Lee et al, 2019 ; Zheng et al, 2019 ; Adedeji et al, 2020 ; Li et al, 2020b ), and n-type semiconducting metal oxides, e.g., ZnO ( Upama et al, 2017 ; Ahmad et al, 2019 ; Frankenstein et al, 2019 ; Zhang X. et al, 2020 ; Usmani et al, 2021 ) and TiO 2 ( Lin et al, 2013 ; Sun et al, 2016 ; Al-hashimi et al, 2018 ; Abdallaoui et al, 2020 ; Chaudhary et al, 2021 ), have been commonly used to fabricate OSCs. However, Ca and LiF are usually deposited by thermal evaporation in a high vacuum environment at high temperature, which is expensive, complicated and incompatible with flexible devices; hence, making them unfavourable.…”