Interfacial Engineering with Cross-Linkable Fullerene Derivatives for High-Performance Perovskite Solar Cells

Two fullerene derivatives with styryl and oxetane cross-linking groups served as interfacial materials to modify an electron-transporting layer (ETL) of TiO₂, doped with Au nanoparticles, processed under low-temperature conditions to improve the performance of perovskite solar cells (PSC). The cross-linkable [6,6]-phenyl-C₆₁-butyric styryl dendron ester was produced via thermal treatment at 160 °C for 20 min, whereas the cross-linkable [6,6]-phenyl-C₆₁-butyric oxetane dendron ester (C-PCBOD) was obtained via UV-curing treatment for 45 s. Both cross-linked fullerenes can passivate surface-trap states of TiO₂ and have also excellent surface coverage on the TiO₂ layer shown in the corresponding atomic force microscopy images. To improve the crystallinity of perovskite, we propose a simple co-solvent method involving mixing dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) in a specific ratio (DMF/DMSO = 90/10). The fullerene derivative layer between the ETL and perovskite layers significantly improved electron extraction and suppressed charge recombination by decreasing the density of traps at the ETL surface. A planar PSC device was fabricated with the configuration indium tin oxide/TiO₂ (Au)/C-PCBOD/perovskite/spiro-OMeTAD/Au to attain a power conversion efficiency (PCE) of 15.9%. The device performance was optimized with C-PCBOD as an interfacial mediate to modify the surface of the mesoporous TiO₂ ETL; the C-PCBOD-treated device attained a significantly enhanced performance, PCE 18.3%. Electrochemical impedance spectral and photoluminescence decay measurements were carried out to understand the characteristics of electron transfer and charge recombination of the perovskite/TiO₂ samples with and without a fullerene interfacial layer