Positional Methylation Isomer Additives Regulate the Degradation Pathway of Perovskites

Multi-active-site additives with strong reactivity are an effective strategy to continuously improve device efficiency, and also contribute to the long-term stability of perovskite solar cells (PSCs). However, achieving efficient interaction between all active sites and the perovskite remains a critical challenge. Among various structural modifications, methyl substitution, the simplest functional unit in organic chemistry, offers a unique methylation effect that strongly depends on its positional configuration within a molecule, thereby providing a promising approach to modulate additive reactivity. In this study, we designed and investigated two positional methylation isomers, 1-methylhydantoin (1-MH) and 5-methylhydantoin (5-MH), and demonstrate rational positional methylation can significantly enhance the multisite reactivity of additives with perovskite materials. While both isomers contribute to improved device efficiency, they exhibit markedly different effects on device stability. Notably, the incorporation of 5-MH enabled a power conversion efficiency (PCE) of 26.84% (certified at 26.78%), and delivered outstanding operational stability, retaining 92% of its initial PCE after 2000 h under the ISOS-L2 protocol.