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 CH₃NH₃PbI₃ (MAPbI₃) solar cells, to improve the fill factor (FF) and device stability. The graded ETL was made by mixing phenyl-C₆₁-butyric acid methyl ester (PCBM) molecules and C₆₀-diphenylmethanofullerene-oligoether (C₆₀-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 C₆₀-DPM-OE, which was confirmed by XPS depth-profile analysis and an electron energy-loss spectroscope. Comprehensive studies were carried out to explore the characteristics of the graded ETLs in MAPbI₃ solar cells, including the surface properties, electronic energy levels, molecular packing properties and energy transfer dynamics. The elimination of the s-shape in the current density-voltage curves results in an increase in the FF, which originates from the smooth contact between the C₆₀-DPM-OE and hydrophilic MAPbI₃ and the formation of the more ordered ETL. There was an improvement in device stability mainly due to the decrease in the photothermal induced morphology change of the graded ETLs fabricated from two fullerene derivatives with distinct hydrophilicity. Consequently, such a graded ETL provides dual-functional capabilities for the realization of stable high-performance MAPbI₃ solar cells.