Lead halide perovskite solar cells (PSCs) have drawn great attention as the promising solar energy source in recent years. Their certified power conversion efficiencies (PCEs) now reach more than 20%. Despite high PCE, low stability of PSCs has been the most serious drawback, which is in dire need of resolution. It has been found that light, oxygen, and water are the origins of the perovskite layer degradation. In particular, dopants used for 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobi-fluorene (spiro-MeOTAD) hole-transporting material are attributed for the low stability, due to their hygroscopic nature. Replacement of liquid electrolyte by spiro-MeOTAD was a breakthrough at the initial stage of PSC development, improving PCE substantially, owing to well-aligned energy band and amorphous nature of spiro-MeOTAD. However, because spiro-MeOTAD intrinsically has low mobility, a dopant must be present to improve the conductivity. Lithium bis(trifluoromethanesulfonyl)imide salt (Li⁺TFSI⁻) has been widely used as the p-dopant for spiro-MeOTAD. Li⁺TFSI⁻ does not directly oxidize spiro-MeOTAD, but instead, promotes the oxidation of spiro-MeOTAD by oxygen at the presence of light or thermal excitation. Because doping spiro-MeOTAD with Li⁺TFSI⁻ requires oxygen, it is difficult to control the amount of spiro-MeOTAD oxidation; the formation of oxidized spiro-MeOTAD has been shown to be reversible during device operation depending on ambient conditions and the sweep direction of current−voltage (J−V) measurement. Also, a number of factors, including light intensity, the concentrations of lithium ions and oxygen, has been shown to the amount of oxidized spiro-MeOTAD. Uncontrolled oxidized spiro-MeOTAD is unpredictable and leads to inconsistent reproducibility and instability of PSCs. In addition to this, as mentioned above, Li⁺ salt is extremely hygroscopic and induces moisture-based degradation of PSCs.

In this work, lithium-ion-encapsulated [60]fullerene bis(trifluoromethanesulfonyl)imide salt ([Li⁺@C₆₀]TFSI⁻) was used in PSCs instead of Li⁺TFSI⁻ as a solution aforementioned problems. C₆₀ encapsulating Li⁺ changed the hydrophilic alkali salt to a hydrophobic species. Also, spiro-MeOTAD mixed with [Li⁺@C₆₀]TFSI⁻produced cationic salt spiro-MeOTAD^●+TFSI⁻ and neutral Li⁺@C₆₀^●− by electron transfer from spiro-MeOTAD to Li⁺@C₆₀. While spiro-MeOTAD^●+TFSI⁻functioned as an effective hole transporting material Li⁺@C₆₀^●− functioned as an antioxidant, reacting with any intruding oxygen and moisture. spiro-MeOTAD^●+TFSI⁻ required no chemical additives nor oxidation as it was technically pre-oxidized spiro-MeOTAD. By preventing unnecessary oxidation in the device system, [Li⁺@C₆₀]TFSI⁻-used devices showed approximately 10 times higher stability than the conventional Li⁺TFSI⁻-used devices.