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

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.