Bifunctional Tetramethylammonium Acetate Modifier Synchronizes Defect Passivation and Energy-Level Alignment for HTL-Free Carbon-Based Inorganic Perovskite Solar Cells

Abstract Carbon-based HTL-free CsPbI2.75Br0.25 all-inorganic perovskite solar cells face challenges from interfacial defects and suboptimal energy-level alignment. In this work, tetramethylammonium acetate (TMMAc) is employed for the first time as a multifunctional surface modifier to concurrently address these issues. TMMAc features a unique zwitterionic structure that enables synergistic passivation: its acetate anion (CH3COO–) acts as a strong Lewis base, exhibiting strong interaction with uncoordinated Pb2+ ions, thereby effectively passivating lead-related defects. Meanwhile, its tetramethylammonium cation ((CH3)4N+) passivates halogen vacancies. Moreover, the intrinsic strong polarity of TMMAc and the formed oriented dipole layer at the interface modulate the surface work function, inducing a favorable gradient energy-level alignment between the perovskite and the carbon electrode, which significantly reduces the hole extraction barrier. As a result, the TMMAc-modified device achieves a PCE of 17.93% and an open–circuit voltage of 1.19 V, along with markedly improved operational stability.