Electron-donor/-acceptor ratio-guided molecular engineering for buried interface optimization in n-i-p perovskite solar cells

The buried interface between the electron transport layer (ETL) and perovskite is critical for the performance of perovskite solar cells (PSCs). Modifying the microstructure of this buried interface using dipolar molecules is among the most effective strategies to enhance device performance. However, the influence of the electron-donating/electron-withdrawing group ratio (EDG/EWG ratio) of dipolar molecules on buried interface engineering has not been systematically investigated. In this work, dipolar molecules are classified into EWG-rich, balanced, and EDG-rich configurations according to their EDG/EWG ratio, using L-aspartic acid (AA), 4-aminobutyric acid (ABA), and L-2,4-diaminobutyric acid (DBA) as model systems. We confirm that the primary factor limiting device performance is located on the perovskite side rather than the ETL side. Both experimental and theoretical results reveal that the EDG-rich dipolar configuration provides the most efficient defect passivation for perovskite, promotes the growth of high-quality perovskite films, strengthens the interfacial electric field, and accelerates interfacial electron extraction and transport. As a result, the DBA-modified device achieves a champion PCE of 24.18% and maintains 85% of its initial efficiency after 30 days of ambient storage (20-25 °C, 25%-30% relative humidity) without encapsulation, showing excellent long-term stability. This work establishes asymmetric molecular engineering as a key design principle for optimizing the buried interface in high-performance PSCs.