Highly thermal-stable perylene-bisimide small molecules as efficient electron-transport materials for perovskite solar cells

Perylene-bisimide (PDI)-based small molecules (PDI-Ph, PDI-PhCN, PDI-PhCN-2Br and PDI-PhCN-4Br) were synthesized via imidization of perylene bisanhydride and core-bromided perylene bisanhydride. The physical, optical and electronic properties of these molecules were characterized by thermogravimetric analysis (TGA), UV-Vis, X-ray diffraction (XRD), cyclic voltammetry and space charge-limited current (SCLC). PDI-Ph, PDI-PhCN and PDI-PhCN-2Br show excellent thermal stability with decomposition temperatures above 400 °C (610 °C for PDI-PhCN) and high crystallinity with strong π-π stacking. The three molecules also exhibit high electron mobility with average values of 0.169 cm2 V-1 s-1 for PDI-Ph, 0.212 cm2 V-1 s-1 for PDI-PhCN and 0.119 cm2 V-1 s-1 for PDI-PhCN-2Br. Utilizing these molecules as the single electron-transporting layer (ETL), inverted perovskite solar cells with a configuration of ITO/NiOx/MAPbClxI3-x/ETL/Ag were fabricated. A power conversion efficiency of 14.6% was achieved from the device using PDI-PhCN as ETL. Furthermore, when BCP was used as the hole-blocking layer, the identical structured perovskite device achieved a high efficiency of 18.8% for the PDI-PhCN/BCP combination, which was better than the standard cell (17.4%) using C60/BCP as ETL. The superior performance of PDI-PhCN compared to PDI-Ph, PDI-PhCN-2Br and PDI-PhCN-4Br comes from its higher electron mobility and better matched energy levels with that of the absorber MAPbClxI3-x. Our results demonstrate that PDI-based small molecules are very promising electron-transporting materials for highly efficient, low-cost perovskite solar cells.