Thermally Evaporated Naphthalene Diimides as Electron Transport Layers for Perovskite Solar Cells

Thermally evaporated organic electron transport layers (ETLs) have the potential to enable high-performance and scalable perovskite solar cells (PSCs). Among these, naphthalene diimide (NDI)-based ETLs are a promising family of materials that exhibit the optoelectronic properties, ambient stability and versatility required of high-performance ETLs. Here, we synthesized five NDI derivatives with varying functional groups and identified the two most promising candidates for evaluating the impact of molecular structure on processability via thermal evaporation. While phosphonic acid functionalization was shown to introduce thermal instability, leading to chemical changes during evaporation, NDI-bis N-phenyl-bromide (NDI-(PhBr)₂) emerged as a promising ETL candidate. NDI-(PhBr)₂ demonstrated excellent compatibility with the thermal evaporation process and enabled PSCs with power conversion efficiencies (PCEs) of 15.6%, surpassing all previously reported PSCs containing thermally evaporated NDI ETLs. Furthermore, NDI-(PhBr)₂ exhibited excellent operational stability, retaining 75% of the initial PCE after 150 h of operation under continuous illumination at 65 °C. These results highlight the potential of NDI-based ETLs for advancing the scalability and performance of PSCs.