Functional graded fullerene derivatives for improving the fill factor and device stability of inverted-type perovskite solar cells.

A graded fullerene derivative thin film was used as a dual-functional electron transport layer (ETL) in CHNHPbI (MAPbI) solar cells, to improve the fill factor (FF) and device stability. The graded ETL was made by mixing phenyl-C-butyric acid methyl ester (PCBM) molecules and C-diphenylmethanofullerene-oligoether (C-DPM-OE) molecules using the spin-coating method. The formation of the graded ETLs can be due to the phase separation between hydrophobic PCBM and hydrophilic C-DPM-OE, which was confirmed by XPS depth-profile analysis and an electron energy-loss spectroscope.