Conjugated ionic porphyrins as the cathode interlayer materials in organic solar cells

Abstract

Abstract Cathode interlayer materials (CIMs) play very important roles in organic solar cells (OSCs). In this study, we synthesize three porphyrin CIMs Br−FNEZnP-OE, Br−FNEZnP-TC16 and SO3−FNEZnP-TC16 with the same conjugated backbones but different ionic side chains at the fluorene units and different substituents at the porphyrin cores. Because Br−FNEZnP-TC16 and SO3−FNEZnP-TC16 are substituted with hydrophobic alkyl-thienyls while Br−FNEZnP-OE contains more hydrophilic di-ethanediol ester phenyls at the porphyrin cores, Br−FNEZnP-TC16 and SO3−FNEZnP-TC16 are soluble in methanol but insoluble in water while Br−FNEZnP-OE soluble in not only methanol but also water. The bulk heterojunction (BHJ) OSCs based on PTB7:PC71BM (PTB7: poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-di-yl] [3-fluoro-2-[(2-ethylhexy)carbonyl]thienyl[3,4-b]thiophenediyl]], PC71BM: [6,6]-phenyl C71butyric acid methyl ester) with Br−FNEZnP-TC16 and SO3−FNEZnP-TC16 cathode interlayers (CILs) exhibit high power conversion efficiencies (PCEs) of 9.07% and 8.86%, which are enhanced by 66.4% and 62.6%, respectively, with simultaneously improved open circuit voltages, short circuit currents (Jsc) and fill factors compared with those of the devices without any CIL (PCE: 5.45%), and even higher than those of the devices with widely used PFN CILs (PCE: 7.74%) (PFN: poly [(9,9-bis(3´-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)). In contrast, the OSCs with Br−FNEZnP-OE CILs only show a slightly enhanced PCE of 6.55% with a reduced Jsc compared with that of the control devices with no CILs. We also investigate why the performance of the devices with Br−FNEZnP-OE CILs is inferior to those with Br−FNEZnP-TC16 and SO3−FNEZnP-TC16 CILs.