Design Of Small Molecule Conjugated Electrolytes For Polymer Solar Cells

Bulk heterojunction(BHJ) polymer solar cells(PSCs) have attracted a great deal of interest due to their advantages including low cost, lightweight, flexibility and large-scale fabrication through roll-to-roll process. Recently, the record efficiency(10.62%) of single-junction PSCs has been achieved in the inverted device.In the devices structure, interfacial materials play a crucial role in improving the performance of PSCs. A desirable interfacial engineering could tune a better energy alignment and improve the contact interface between the photoactive layer and electrode to promote the charge selective transport and collection, which can avoid the charge carriers reaching the opposite electrodes, effectively. In addition, an excellent interface interlayer could also induce the photoactive layer to generate a perfect morphology, which is significant for the light harvesting, exciton dissociation, charge transport and charge collection in PSCs. Currently, a variety of electron-transporting layer(ETL) materials have been applied in inverted PSCs, including inorganic metal oxides(such as Cs2CO3, TiOx, and ZnO) and organic conjugated polyelectrolytes(CPEs)(such as PFN, PFNBr,and so on). Whereas, the inorganic metal oxides always required high temperature annealing and evaporating processes, which is not helpful for the large area production. As well as the CPEs, it is difficult to design and synthesis with a well-defined structures and a batch-to-batch reproducibility. In view of these problems, the small molecular conjugated electrolytes(SMCE) were draw particular attention due to the advantages of easy purification, monodispersity, well-defined structures and better batch-to-batch reproducibility.In this paper, three fluorene-based alcohol-soluble small molecular conjugated electrolytes(SMCE) 6-(2,7-bis(5-octylthiophen-2-yl)-9-(6-(trimethylammonio)hexyl)-9H-fluoren-9-yl)-N,N,N-trimethylhexan-1-aminium bromide(TFTN-Br), 6-(2-(5-(9,9-bis(6-(trimethylammonio)hexyl)-9H-fluoren-2-yl)thiophen-2-yl)-9-(6-(trimethyl ammonio)hexyl)-9H-fluoren-9-yl)-N,N,N-trimethylhexan-1-aminium bromide(FTFN-Br) and 6-(2-(5-(5-(9,9-bis(6-(trimethylammonio)hexyl)-9H-fluoren-2-yl) thiophen-2-yl)thiophen-2-yl)-9-(6-(trimethylammonio)hexyl)-9H-fluoren-9-yl)-N,N,N-trimethylhexan-1-aminium bromide(FTTFN-Br), bearing amine groups on side chains, are designed and synthesized via Stille coupling reactions. They can be dissolved in methanol(MeOH), dimethyl formamide(DMF), dimethylsulfoxide(DMSO) and other polar solvents, which can avoid the erosion from the upper organic active layer as cathode interfacial layers in the inverted polymer solar cells(I-PSCs). Inspiringly, the devices based TFTN-Br, FTTFN-Br and FTFN-Br as cathode interfacial layers possessed a better performance than the common I-PSCs based ZnO cathode interfacial layer. What’s more, the FTTFN-Br based device with a power conversion efficiency(PCE) of 3.5% compare to that of the FTFN-Br based device with a PCE of 3.1%, which probably owe to a better film forming caused by the adding thiophene unit in the conjugated backbone. While it’s surprised that the device based TFTN-Br with a shorter conjugated backbone showed a best PCE of 3.8%, which presumably ascribe to the C8 alkyl chains in the terminal of the conjugated backbone with a strong induction to obtain a favorable morphology of the active layer so as to improve the device performance.