Interface Engineering of Air-Stable n-Doping Fullerene-Modified TiO₂ Electron Transport Layer for Highly Efficient and Stable Perovskite Solar Cells

As one common electron transport material for planar n-i-p perovskite solar cell, titanium dioxide (TiO₂) compact layer has several challenging issues, such as surface hydroxyl groups, high defect density, and unmatched energy levels, causing severe energy loss and poor stability at contact. To solve these problems, the authors introduce a thin [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) interlayer doped with an air stable n-type dopant, 3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (DMBI) to modify the TiO₂ surface. The state-of-the-art characterizations demonstrate such modification significantly improves charge transfer at MAPbI₃/TiO₂ interface together with smaller energy level offset, leading to suppressed charge recombination. High-quality perovskite film with larger crystal grain size grows on the n-doped PCBM/TiO₂ attributed to the better surface affinity. As a result, the average power conversion efficiency of perovskite solar cell exhibits a prominent improvement from 17.46% to 20.14%, with an enhancement in all device photovoltaic parameters. In addition, the stability of the device with n-doped PCBM/TiO₂ is much better than that of the control device with the bare TiO₂ due to hydrophobicity nature of PCBM and low defect densities in the perovskite film and at the interface. This work indicates that many further device performance improvements should be conceivable by focusing on the perovskite interface.