Increasing the Li-TFSI doping concentration in Spiro-OMeTAD enables efficient and stable perovskite solar cells

Abstract

Li-TFSI/tBP is a classic doping system for Spiro-OMeTAD in typical n–i–p structure perovskite solar cells (PSCs). Unfortunately, the solubility of Li-TFSI in chlorobenzene is limited, and tBP needs to be added to promote its dissolution. However, tBP is a volatile polar solvent that can damage perovskites and Spiro-OMeTAD films. Moreover, even with the addition of tBP, the maximum concentration of Li-TFSI in chlorobenzene is basically 50 mol%, and the free radicals generated by doped Spiro-OMeTAD are limited. If the Li-TFSI concentration is further increased, it will precipitate and hinder the doping effect. Herein, we demonstrated an effective strategy to improve the performances of the PSCs by enhancing the solubility of Li-TFSI and increasing its doping concentration from 50 mol% to 80 mol% through the substitution of tBP with 12-crown-4. The chelation of 12-crown-4 with Li⁺ not only increased the solubility of Li-TFSI in chlorobenzene and enhanced its doping efficiency but also effectively addressed issues such as Li⁺ migration, hygroscopicity, and pinholes caused by the Li-TFSI/tBP system. The PSCs based on this strategy achieved a champion power conversion efficiency (PCE) of 23.99% and maintained 83% of the initial PCE under ISOS-D-3 protocol aging for 30 days.