Lithium‐Based Deep Eutectic in Spiro‐OMeTAD Enable Efficient and Stable Perovskite Solar Cells

ABSTRACT Doping of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4‐ tert ‐butylpyridine in 2,2',7,7'‐tetrakis( N,N ‐di‐p‐methoxyphenylamine)‐9,9'‐spirobifluorene (Spiro‐OMeTAD) is essential for achieving high efficiency in n‐i‐p type perovskite solar cells (PSCs). However, the migration of Li + and hygroscopic nature of LiTFSI reduce the long‐term stability of PSCs. In this work, we select 2,2’‐dipyridyl disulfide (DpyDS) to interact with Li + and it can form a liquid deep eutectic mixture with LiTFSI at room temperature. The liquid molecular film exhibits a dual function of suppressing Li + diffusion and repairing pores within the hole transport layer. In addition, DpyDS can facilitate the dissociation of TFSI − and enhance the hole mobility of Spiro‐OMeTAD, thereby promoting hole extraction and reducing non‐radiative recombination losses. The device doped with DpyDS achieves an unprecedented power conversion efficiency (PCE) of 26.53% (certified 26.37%). After 1800 h of maximum power point tracking, the unencapsulated device retains 90.5% of its initial PCE under the ISOS‐L‐1I protocol. This work validates a promising deep eutectic strategy with LiTFSI in Spiro‐OMeTAD, enhancing the PCE and long‐term stability of PSCs.