Bilayered Molecular Bridge Mediated by π–π Stacking for Improved Interfacial Charge Transport in Perovskite Solar Cells

Molecular bridges with one end absorbed on the electron transport layer (ETL) and the other bound to perovskite can effectively repair imperfections at the ETL/perovskite interface. However, single-layered bridges usually coexist with undesired double-layered molecules, leaving a Van der Waals gap between them. Charge transport can only occur via the tunneling effect to travel through the gap, which requires a forward voltage bias and leads to a constrained charge transport efficiency. Herein, the study designs and synthesizes an imidazolium derivative ionic salt of 1,3-dibenzyl-2-phenylimidazolium chloride (DPhImCl), featuring multiple aromatic side chains, to form bilayered interfacial molecular bridges mediated by π–π stacking. The study reveals that DPhIm⁺ strongly adsorbs on both the SnO₂ and perovskite surfaces via the imidazolium ring, while the two layers of DPhIm⁺ absorbed on SnO₂ and perovskite respectively interact through π–π stacking of the benzene ring in side chains, forming bilayered molecular bridge at the SnO₂/perovskite interface. This π–π interaction promotes the orderly stacking of molecular layers and creates hopping channels for electron transport, thus facilitating the interfacial charge transfer efficiency. As a result, an impressive device efficiency of 25.90% (certified 25.27%) and a robust T₉₀ operational lifetime of 1101 h for n-i-p perovskite solar cells achieved.