Defect Healing of Electron Transport Layers Using Multifunctional Carboxyl Acid for Carbon‐Based Flexible Perovskite Solar Cells

ABSTRACT Defects in electron transport layers (ETLs) critically limit the performance of perovskite solar cells, particularly in flexible architectures. Here, we introduce a defect healing strategy using multifunctional carboxylic acids to modify SnO 2 ETLs on flexible substrates. Theoretical calculations reveal strong adsorption energies between carboxylate groups and SnO 2 surface atoms, enabling effective coordination with undercoordinated Sn sites and oxygen vacancies. Density functional theory screens that 2‐bromo‐1,3,5‐benzenetricarboxylic acid (BBA) exhibits the highest binding energy owing to synergistic Sn–Br interactions, which enhance interfacial coupling and electron transfer. The optimized interface between SnO 2 and BBA facilitates uniform perovskite film growth and suppresses trap‐assisted recombination. Consequently, the carbon‐based flexible perovskite solar cells deliver an efficiency of 17.07% and retain 68% of the initial efficiency after 30 days without encapsulation. This work establishes a versatile and low‐temperature strategy for defect regulation in SnO 2 ETLs, offering a promising pathway toward efficient and durable flexible perovskite photovoltaics.