Buried Interface Engineering via a Bifunctional Guanidino‐Carboxyl Molecular Bridge for Enhanced Performance and Stability in Perovskite Solar Cells

ABSTRACT Precise regulation of the buried interface between the electron transport layer (ETL) and the perovskite absorber is crucial for achieving highly efficient and stable perovskite solar cells (PSCs). In this work, we introduce 4‐guanidinobutanoic acid (GCC) as an interfacial modifier to construct a functional molecule‐bridge structure at the SnO 2 /perovskite interface. Utilizing GCC at the buried interface enables molecular‐level passivation and effectively suppresses the interfacial defect‐state density. For the SnO 2 ETL, GCC passivates oxygen vacancies, enhances electrical conductivity, and tunes the energy levels of SnO 2 to better match those of the perovskite. In addition, GCC improves substrate wettability and thereby regulates perovskite crystallization, leading to high‐quality films with reduced defects and mitigated nonradiative recombination. As a result of these combined benefits, the champion device (active area of 0.09 cm 2 ) achieves a power conversion efficiency (PCE) of 25.37%. Importantly, when the device area is scaled up to 13.4 cm 2 , an outstanding PCE of 20.01% is still obtained. Furthermore, this molecule‐bridge strategy significantly improves the stability of the PSCs against moisture, heat, and light‐induced degradation, indicating the prospect of industrial applications.