Bifunctional Urea–Polyethyleneimine‐Mediated Surface Engineering in SnO2 Electron‐Transport Layer for Efficient and Stable Organic Solar Cells

Because of its high conductivity, wide bandgap, and excellent photostability, tin oxide (SnO 2 ) has long been recognized as an electron‐transport layer (ETL) in organic solar cells (OSCs). However, the energy‐level mismatch between the work function (WF) of SnO 2 and the lowest unoccupied molecular orbital level of Y‐series nonfullerene acceptors (NFAs), along with the abundance of surface defects on SnO 2 , have limited its widespread application as ETLs in OSCs. Herein, a novel approach utilizing urea‐functionalized polyethyleneimine (PEI) materials called u ‐PEIs for modifying SnO 2 is introduced. This modification, which serves dual purposes of WF modulation and surface‐defect passivation, can mitigate the energy barriers of SnO 2 /Y‐series NFA and increase the conductivity of the SnO 2 film. PM6:Y6‐based OSCs with u ‐PEI‐modified SnO 2 (SnO 2 : u ‐PEI) ETLs exhibit a remarkable efficiency of 16%, which significantly exceeds that (13.5%) achieved with bare SnO 2 ‐based OSCs, along with outstanding photo‐ and thermal stability. This study confirms the efficacy of urea‐functionalized PEI for efficient and stable OCSs, paving the way for SnO 2 applications.