Impact of Functional Group Configuration in Isomeric Additives on Device Performance of Quasi-2D Perovskite Solar Cells

Additive engineering has shown great potential in modulating crystallization kinetics and reducing defects in quasi-2D perovskite films. However, most studies have primarily focused on the types of functional groups, while the influence of their spatial configuration remains largely overlooked. Here, we systematically investigate the impact of functional group configuration on quasi-2D perovskite solar cells using an isomeric molecular pair, cytosine and iso-cytosine, as a model system. Despite sharing identical functional groups, their distinct spatial configurations lead to different charge distributions and interactions with the perovskite components. Consequently, cytosine exhibits stronger and more delocalized interactions that promote favorable nucleation and crystallization, yielding more ordered film structures, whereas iso-cytosine shows comparatively weaker and more localized interactions. As a result, cytosine-based devices achieve a champion power conversion efficiency of 22.4% and demonstrate excellent thermal stability, with over 80% of the initial performance retained after 3600 h of thermal aging at 60 °C.