Fluorene dimers as the cathode interlayers in organic solar cells

Abstract

Abstract Cathode interlayers (CILs) play important roles in enhancing the performance of organic solar cells (OSCs). Though small molecule cathode materials (CIMs) are easier to be synthesized and optimized than polymer CIMs, the devices with SM CILs usually show inferior performance than the devices with polymer CILs. In this study, we synthesized two (FN) dimers FN-FN and (FN-E)2 (FN: 3,3′-(9H-fluorene-9,9-diyl)-bis-(N,N-diethylpropan-1-amine)) connected directly and by 1,4-butadiyne, respectively, by only three reaction steps as the CILs in OSCs. And the bulk-heterojunction (BHJ) solar cells based on PTB7:PC71BM) active layers (PTB7: poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’] dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophene-diyl]], PC71BM: [6,6]-phenyl C71 butyric acid methyl ester with FN-FN and (FN-E)2 CILs show enhanced power conversion efficiencies (PCEs) of 8.42% and 9.02%, which are increased by 45% and 56%, respectively, compared with that of the control devices without any CIL (PCE: 5.78%). And these PCEs are even higher than the PCE (7.50%) of the devices with widely used polymer PFN CILs (PFN: (poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) due to the better cathode modification capability of FN-FN and (FN-E)2. Furthermore, we also investigated why the OSCs with (FN-E)2 CILs show higher performance than those with FN-FN CILs.