Amino N-oxide functionalized graphene quantum dots as a cathode interlayer for inverted polymer solar cells

Solution-processable graphene derivatives, such as graphene oxide (GO) and graphene quantum dots (GQDs), are a promising class of cathode interlayer (CIL) materials for polymer solar cells (PSCs). These graphene derivatives always contain ions, which may migrate under electrical field during the PSC device operation and consequently deteriorate device stability. In this manuscript, by functionalizing GQDs with amino N-oxide groups at the edge of the graphene basal plane, we develop a non-ionic graphene derivative (GQD-NO) as a CIL. The periphery polar amino N-oxide groups form the desired interfacial dipole with an indium tin oxide (ITO) electrode to lower the work function by as much as 1.06 eV. As a result, GQD-NO as a CIL forms Ohmic contact with the active layer of PSC devices, improves electron transport/extraction, blocks holes and prevents charge recombination on the cathode. Owing to the periphery amino N-oxide groups, GQD-NO exhibits good solubility in water and insolubility in common low-polarity organic solvents, which enables multilayer inverted PSC device fabrication. In addition, the small lateral size of GQD-NO leads to a large bandgap and transparency in the visible/NIR region. Using a blend of poly[N-9-hepta-decanyl-2,7-carbazole-alt-5,5-(4,7-di-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the active layer, the inverted PSC device with GQD-NO as CIL shows the power conversion efficiency of 7.43%, which is much higher than that with the state-of-the-art ZnO as CIL (PCE = 6.56%) or without the CIL (PCE = 3.16%). These results indicate that non-ionic GQD-NO is a promising CIL for PSCs.