Concise synthesis of low-cost fullerene derivatives as electron transport materials for efficient air-processed invert perovskite solar cells

Due to its excellent charge extraction ability, fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) is widely used as electron transport materials (ETM) in invert perovskite solar cells. However, the complicated synthetic routes and low productivity of PCBM limiting its commercial application. Moreover, the insufficient defect passivation ability of PCBM is contributed to inferior device performance because it lacks hetero-atoms/groups with lone pair electrons, it is highly desirable for exploration of new fullerene-based ETM with excellent photoelectric properties. Therefore, three new fullerene malonate derivatives were synthesized by simple two-step reaction with a high yield, and then developed as electron transport materials in invert perovskite solar cells which fabricated in air condition. The constituent thiophene and pyridyl group of the fullerene-based ETM can heighten the chemical interaction between under-coordinated Pb2+ and lone pair electrons of N, S atom by electrostatic interaction. Hence, the air-processed unencapsulated device with new fullerene-based electron transport materials (C60-bis(pyridin-2-ylmethyl) malonate (C60-PMME)) can obtain a enhanced power conversion efficiency (PCE) of 18.38%, which is significantly higher than the PCBM-based devices (16.64%). Additionally, the C60-PMME-based devices exhibit significantly more outstanding long-term stability than PCBM-based devices, owing to the strong hydrophobic properties of these new fullerene-based ETM. This study shows the promising potentials of these new low-cost fullerene derivatives as ETM to replace commercially used fullerene derivatives PCBM.