Design of Ionic Lock Structure for a tBP-Free and Green-Solvent-Processed Hole Transporter in Perovskite Solar Cells

Perovskite solar cells (PSCs) employing 2,2″,7,7″-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’-spirobifluorene (Spiro-OMeTAD) as the hole-transporting material (HTM) often suffer from dopant-induced instabilities, including lithium (Li) salt aggregation, ion migration, and the volatilization of tert-butylpyridine (tBP). Additionally, the solution processing of conventional HTMs typically relies on toxic solvents, posing environmental and industrial challenges. In this study, we introduce an “ionic lock” strategy by incorporating polyoxygenated side chains─such as crown ether and 2-methoxyethoxy groups─into the backbone of conjugated hole conductors. The resulting HTMs, MPA-NDTCE and MPA-NDTME, exhibit excellent solubility in eco-friendly solvents like 3-methylcyclohexanone (3-MC) and demonstrate favorable compatibility with Li-based dopants. Notably, MPA-NDTCE shows enhanced molecular ordering, superior hole mobility, and stronger interactions with Li+ ions, facilitating dopant immobilization and stabilizing the film morphology. Consequently, PSCs utilizing MPA-NDTCE as a green-solvent-processable HTM achieve a high power conversion efficiency of 22.6%, coupled with robust thermal stability under aging at 65 °C. These improvements are attributed to the elimination of tBP and the effective suppression of Li-ion migration.