Reducing Photovoltage Loss in Highly Efficient p–i–n Perovskite Solar Cells via Alkyl Chain Engineering

ABSTRACT Surface defect passivation using organic ammonium salts (OASs) has become a widely adopted strategy in perovskite solar cells (PSCs). However, the influence of the alkyl chain structure in OASs on the interfacial energy alignment and defect passivation mechanism remains largely unexplored. In this work, we systematically investigate two diammonium iodide salts, 1,4‐butanediammonium diiodide (BDADI, linear) and 1,4‐piperidinium diiodide (PDI, cyclic), to elucidate their effects on perovskite surface passivation, energy‐level alignment, and photovoltage loss. Structural and electronic analyses reveal that PDI induces the in situ formation of a well‐defined 2D/3D perovskite heterojunction at the surface, attributed to its more delocalized electron distribution within the cyclic alkyl backbone compared to the localized electronic structure of the linear BDAD + cation. PDI‐modified devices exhibit reduced trap‐state density, optimized band alignment, and enhanced carrier transport. Consequently, the PDI‐passivated PSCs achieve a remarkable power conversion efficiency of 26.0% (25.74% certified) with a minimized photovoltage loss of 0.36 eV, while maintaining 95% of their initial efficiency after 1000 h of unencapsulated storage, which exhibits outstanding performance among the reported values for applying OASs on the perovskite surface in PSCs. This work demonstrates an effective molecular‐engineering approach to modulate interfacial properties via alkyl chain design.