Pyridine-incorporated alcohol-soluble neutral polyfluorene derivatives as efficient cathode-modifying layers for polymer solar cells

A class of novel alcohol-soluble conjugated polymers with pyridine incorporated at the side chains of fluorene scaffolds is developed and used as cathode interfacial layers (CILs) for both conventional and inverted polymer solar cells (PSCs). The pyridine group can endow these polymers with solubility in methanol (MeOH) in the presence of trace acetic acid (AcOH), which is beneficial for the realization of PSC fabrication by low-cost solution processing without interfacial mixing. In addition, the pyridine group can not only build interfacial dipoles at the CIL/cathode interfaces, leading to reduced cathode work functions and improved open-circuit voltages, but also n-dope the fullerene acceptors, which decreases the interfacial energy loss at the cathode side. As a result, for conventional PSCs based on poly[ N -9′′-heptadecanyl-2,7-carbazole- alt -5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM), these resulting polymers can increase the power conversion efficiency (PCE) from 5.38% (without any CIL) and 5.50% (with MeOH/AcOH treatment) to 6.54%–6.97%. For the inverted PSCs based on poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2- b :4,5- b ′]dithiophene-2,6-diyl- alt -ethylhexyl-3-fluorothieno[3,4- b ]thiophene-2-carboxylate-4,6-diyl] (PTB7):PC71BM, these resulting polymers can improve the PCE from 3.64% (without any CIL) and 4.25% (with MeOH/AcOH treatment) to 7.21%–7.96%. Therefore, these pyridine-functionalized polymers are promising candidates as CILs for high-performance PSCs.