A Plant‐Derived Bridge Multifunctionalizes Buried Interfaces for Ambient‐Air‐Fabricated Flexible Perovskite Solar Cells with High Efficiency and Stability

Flexible perovskite solar cells (f‐PSCs) have garnered significant interest due to their lightweight design, high power‐to‐weight ratio, and potential applications in wearable electronics. However, critical challenges such as SnO 2 nanocrystal aggregation, buried interface defects, and inadequate mechanical durability hinder the optoelectronic performance and operational stability of f‐PSCs. This study proposes a bottom‐up cross‐layer engineering strategy that pre‐embeds a multifunctional plant‐derived molecule—the potassium salt of DL‐theanate (P‐DLTh)—into the SnO 2 electron transport layer. P‐DLTh is chemically adsorbed onto the SnO 2 surface, enabling the suppression of nanoparticle aggregation, modulation of charge transport properties, mitigation of interfacial residual stress, and optimization of the perovskite crystallization kinetics. Through synergistic coordination bonds, ionic interactions, and hydrogen bonding, this approach establishes a triple‐layer passivation system addressing the defects in the SnO 2 bulk, surface, and buried perovskite interface. Leveraging these mechanisms, the ambient‐air‐fabricated flexible device achieves an optimal efficiency of 24.45%, which is particularly impressive for air‐processed f‐PSCs. This work pioneers a natural‐molecule‐based cross‐layer modulation strategy for advanced optoelectronics.