Abstract
Perovskite solar cells (PSCs) have been widely studied and such research resulted in high power conversion efficiency (PCE). However, stability remains a great challenge when TiO2 or other wide bandgap n-type oxide semiconductors are used as the electron transport layer (ETL). The use of carbon such as fullerene (C-60) as an ETL has shown promise with much improved UV stability. But there are two major obstacles to overcome: the first is the difficulty to fabricate C-60 films with full coverage mainly due to the low solubility of fullerene in dichlorobenzene and the second is the hydrophobicity of C-60 that hinders the deposition of pin hole free perovskite films with high crystallinity, intimate contact, and a desired columnar microstructure. In this work, a C-60/ultrathin-TiOx (u-TiOx) bilayer is designed and fabricated as a compact ETL, which restrains the charge recombination at the ETL/perovskite interface and significantly enhances the PSC UV stability. Not only does the introduction of TiOx on top of the C-60 film fill the holes or gaps in the C-60 film, but also it enhances the surface energy benefiting the growth of perovskite with an intimate contact between the ETL and perovskite, which results in an enhanced perovskite crystallization and a reduced charge recombination at the interface. Both the open-circuit voltage and fill factor were largely improved to obtain a PCE of 19.38% with a rigid device. The highest efficiency 14.74% of a larger-area flexible PSC (1.0 cm(2)) based on the bilayer was obtained due to the superior homogeneity of the films. More importantly, by eliminating the negative charge accumulation at the perovskite/ETL interface and suppressing the irreversible moisture-driven decomposition of perovskite materials, the C-60/u-TiOx bilayer-based PSC shows outstanding stability, retaining 83% and 90% of its initial performance after 312 h UV irradiation and 1000 h exposure to ambient air, respectively.