Suppressing Li⁺ Migration from Li-TFSI-Doped HTLs via a PEI–Crown Ether Complex for Enhanced Long-Term Stability of Perovskite Solar Cells

Perovskite solar cells (PSCs) are attracting attention as next-generation solar cells due to their lower manufacturing costs and the feasibility of solution processing compared to conventional silicon solar cells. However, they face limitations in long-term stability because of their sensitivity to moisture and oxygen. In particular, Spiro-OMeTAD, which is widely used in n-i-p structured PSCs, requires Li-TFSI doping due to its low conductivity. Unfortunately, the resulting Li⁺ ions act as a major cause of device degradation due to their strong hygroscopicity and high mobility.

In this study, to control the behavior of Li⁺ ions, we synthesized a PEI-tethered crown ether (PEI-FB12C4-H) and applied it to the Spiro-OMeTAD hole transport layer (HTL). The crown ether structure effectively suppressed ion migration through selective coordination bonding with Li⁺ ions. Simultaneously, the secondary and tertiary amines present in the PEI backbone absorbed acidic species (H⁺) and became protonated, thereby inhibiting the formation of HTFSI and mitigating the acidification of the interface. This dual-ion stabilization strategy plays a role in suppressing degradation caused by both moisture ingress and ion-induced processes.

The behavior of Li⁺ ions was confirmed through ToF-SIMS analysis, and the chemical interactions and protonation behavior were investigated using XPS and FTIR analysis. In addition, long-term operational stability, which was improved compared to the reference device, was verified through MPPT measurements. This study presents a novel interfacial stabilization strategy capable of dual-controlling the degradation mechanisms associated with ion migration and acidic species.