Decoupling Bidirectional Photochemical Degradation via Radical Scavenging for Stable Inverted Perovskite Solar Cells.

Interfacial instability between perovskite absorbers and fullerene-based electron transport layers critically limits the operational stability of inverted perovskite solar cells (PSCs). Here, we discover a bidirectional coupling degradation mechanism at the perovskite/fullerene interface: photo-oxidation of formamidinium iodide (FAI) generates iodine radicals that catalyze PCBM dimerization via [2+2] cycloaddition, while PCBM concurrently accelerates FAI deprotonation and iodine-species formation, creating a self-reinforcing degradation cycle. To disrupt this cascade, we introduce the nitroxide radical scavenger 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy radical (O-TEMPO) at the interface, which selectively quenches iodine and carbon-centered radicals, suppressing both perovskite decomposition and PCBM dimerization while maintaining optimal charge extraction. O-TEMPO-modified devices achieve a champion efficiency of 26.99% and retain 95.1% of initial performance after 1000 h of maximum power point tracking under ISOS-L-2 conditions (65°C), significantly outperforming control and conventionally 3-(methylthio)propylammonium iodide (3MTPAI)-passivated devices. This work elucidates the molecular origins of interfacial degradation and establishes radical-scavenging interfacial engineering as a universal strategy to decouple synergistic degradation pathways, providing a robust framework for developing highly stable perovskite photovoltaic technologies.