Electron-Accepting PDI–Cb Interlayer for over 22% Inverted Perovskite Solar Cells with Photo- and Thermal Stability

To investigate the synergistic effect of perylene diimide (PDI) unit and diphenyl-o-carboranyl (Cb) group, PDI–Cb with Cb groups at the bay positions of the PDI unit is synthesized. By introducing 3D carboranyl group into the bay position, PDI–Cb shows distorted geometry between PDI core and the adjacent phenyl ring of Cb due to the ring torsions, which can suppress the aggregation tendency. Furthermore, the proper lowest unoccupied molecular orbital (LUMO) energy level of −4.12 eV may be beneficial to extract electrons from the perovskite efficiently. By introducing PDI–Cb interlayer at perovskite/C60 interface, the inverted perovskite solar cells (PSCs) have significantly enhanced efficiency from 19.98% to 22.31% at 1 sun condition (AM1.5G 100 mW cm⁻²) because the PDI–Cb interlayer promotes charge extraction thanks to the electron-accepting ability of Cb group; thereby, it reduces carrier recombination significantly. In addition, the unencapsulated inverted PSC with the PDI–Cb interlayer has good photo- and thermal stability because it shows 9.3% efficiency degradation after continuous 1 sun light soaking for 1000 h at 85 °C in N₂ atmosphere.