Thickness-insensitive, cyano-modified perylene diimide derivative as a cathode interlayer material for high-efficiency organic solar cells

Interlayer materials play a crucial role in achieving high efficiency in organic solar cells (OSCs). However, slight increases in film thickness often lead to significant charge accumulation and recombination, presenting a challenge for large-scale OSC device fabrication. Therefore, there is a pressing need for interlayer materials that are insensitive to variations in thickness. In this study, we synthesized a cost-effective cyano-modified perylene diimide (PDI) derivative, PDINBrCN, as an interlayer material. Compared to the analogous PDINBr, the introduction of cyano groups lowers the Lowest unoccupied molecular orbital (LUMO) energy level of the molecule, enhancing electron injection and charge transport efficiency. Additionally, PDINBrCN demonstrates excellent solubility in 2,2,2-trifluoroethanol (TFE) and effectively modifies the electrode work function, facilitating device fabrication through orthogonal solvent processing. When utilized as the cathode interlayer in D18:L8-BO devices, PDINBrCN achieved a high power conversion efficiency (PCE) of 18.83 ​% with a film thickness of 10 ​nm. Importantly, PDINBrCN maintained a PCE of 17.90 ​% even when the film thickness was increased to 50 ​nm. In contrast, the analogous PDI derivatives PDINBr and the star cathode interlayer material anthra [2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H, 9H)-tetrone (PDINN) achieved PCEs of 17.17 ​% and 17.06 ​%, respectively, at the same film thickness. Notably, PDINBrCN maintained a PCE of over 16 ​% even with an interlayer thickness of 80 ​nm, marking one of the best results for small molecule PDI derivatives as cathode interlayer materials at this thickness. Our findings demonstrate that PDINBrCN exhibits excellent processability, electrode work function adjustment capability, and crucially, thickness-insensitive properties. Therefore, PDINBrCN holds promise as an efficient and cost-effective cathode interlayer material, with potential for future commercial applications in OSCs.