Precursor formula engineering enabling high quality solution processed C60 films for efficient and stable inverted perovskite solar cells

• Solution processed C 60 films enabled perovskite solar cell efficiency to over 20% • Super-hydrophobic C 60 films promoted the air-stability of perovskite solar cells. • The key for high quality solution processed C 60 films was elucidated. Solution processed fullerene (C 60 ) films as the electron transport layers (ETLs) promise to substantially reduce the cost of the perovskite solar cells (PSCs). However, the self-aggregation nature of C 60 when dissolved in a solvent with high solubility (e.g., o-dichlorobenzene, o-DCB) makes the efficiency of the resulting PSCs lower than 18%. In this paper, we proposed a novel recipe of C 60 precursor solutions (XP-C 60 ) by using o-xylene (o-X) as the solvent and PCBM solution in chlorobenzene (CB) as the additive, which can simultaneously inhibit the aggregation of C 60 and advocate fast solvent drying. Homogeneous C 60 films of high quality from the new recipe solution can be readily fabricated at room temperatures, advocating the efficiency of the printed PSCs to over 17%. Finally, a champion efficiency of over 20% for the PSCs was achieved which is comparable to that of the PSCs with PCBM as the ETLs. XP-C 60 based PSCs also show good stability, which sustain 85% of its initial efficiency as being exposed in air with a relative humidity (RH) of 50% for over 20 days. Our work helps to understand the film-formation dynamics of C 60 by a solution method and cut the production cost of the PSCs to the bone.