Restricting Li⁺ Migration via Mixed Hole Transporting Materials Strategy for Stable tBP-Free Perovskite Solar Cells

Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP) are critical dopants in hole transporting materials (HTMs) for achieving highly efficient n-i-p perovskite solar cells (PSCs). However, the migration of Li⁺ and the volatility of tBP seriously affect the long-term stability of PSCs. In this work, a facile strategy is proposed by mixing a developed terpyridine-based HTM (TPy-CzDPA, simultaneously serving as a tBP alternative) and Li-TFSI into spiro-OMeTAD. Owing to the multidentate structure and strong electrostatic potential of the terpyridine unit, TPy-CzDPA forms a stable coordination with Li⁺, which effectively restricts the migration of Li⁺ and inhibits the associated hydrolysis. Furthermore, replacing tBP with TPy-CzDPA confers not only enhanced thermal stability but also a higher hole mobility of 2.53 × 10^-4 cm² v^-1 s^-1 to the mixed HTMs. Consequently, the PSCs based on mixed HTMs achieve a champion power conversion efficiency (PCE) of 22.13% along with preferable stability. The PSCs with PTAA-based mixed HTMs retain 90% of initial efficiency after 528 h storage at 80°C, while the encapsulated devices exhibit only a 14% drop in PCE after 4800 h storage under 30-40% relative humidity (RH). This work provides a new strategy to promote the performance of tBP-free PSCs.