5-nm LiF as an Efficient Cathode Buffer Layer in Polymer Solar Cells Through Simply Introducing a C₆₀ Interlayer

Lithium fluoride (LiF) is an efficient and widely used cathode buffer layer (CBL) in bulk heterojunction polymer solar cells (PSCs). The LiF thickness is normally limited to 1 nm due to its insulting property. Such small thickness is difficult to precise control during thermal deposition, and more importantly, 1-nm-thick LiF cannot provide sufficient protection for the underlying active layer. Herein, we demonstrated the application of a very thick LiF as CBL without sacrificing the device efficiency by simply inserting a C₆₀ layer between the active layer and LiF layer. The devices with the C₆₀/LiF (5 nm) double CBLs exhibit a peak power conversion efficiency (PCE) of 3.65%, which is twofold higher than that (1.79%) of LiF (5 nm)-only device. The superior performance of the C₆₀/LiF (5 nm)-based devices is mainly attributed to the good electrical conductivity of the C₆₀/LiF (5 nm) bilayer, arising from the intermixing occurred at the C₆₀/LiF interface. Besides, the formation of a P3HT/C₆₀ subcell and the optical spacer effect of C₆₀ also contribute to the increase in short-circuit current density (J _sc) of the device. With further increase of LiF thickness to 8 nm, a PCE of 1.10% is attained for the C₆₀/LiF-based device, while the negligible photovoltaic performance is observed for the LiF-only device. All in all, our results show that C₆₀/LiF bilayer is a promising alternative to LiF single layer due to its high tolerance to the LiF thickness variations.