Universal Metal Electrode Contacts in Perovskite Solar Cells via Fermi‐Level Pinning at Yb‐Doped n + Fullerene Interfaces

Efficient electron extraction at the electron transport layer (ETL)/metal interface is critical for high‐performance perovskite solar cells (PSCs), which is fundamentally governed by interfacial energy‐level alignment; however, the microscopic mechanism by which low‐resistance contacts are established at fullerene/metal interfaces remains poorly understood. Here, we uncover the fundamental electronic mechanism underlying the contact properties of ytterbium (Yb)‐modified fullerene interfaces. By combining quasi‐in situ photoelectron spectroscopy (PES) and reflection electron energy loss spectroscopy (REELS), we found that Yb atoms spontaneously diffuse into the C 60 lattice, forming a well‐defined n + ‐doped surface layer leading to a shallow n–n + homojunction. The n + ‐doped surface drastically reduces the C 60 work function from ∼4.55 eV to 2.50 eV. Crucially, the Fermi level is shifted to above the C 60 lowest unoccupied molecular orbital (LUMO), enabling selection of a variety of metals (Ag, Cu, or Au) without compromising electron extraction. As a result, the Yb‐based PSCs yield consistent high‐power conversion efficiencies (PCEs) across different metal electrodes. This work establishes n + ‐doped surface as an effective method for engineering high‐performance perovskite optoelectronic devices.