Multiple Functionalities of Polyfluorene Grafted with Metal Ion-Intercalated Crown Ether as an Electron Transport Layer for Bulk-Heterojunction Polymer Solar Cells: Optical Interference, Hole Blocking, Interfacial Dipole, and Electron Conduction

We present a novel electron transport (ET) polymer composed of polyfluorene grafted with a K+- intercalated crown ether involving six oxygen atoms (PFCn6:K+) for bulk-heterojunction polymer solar cells (PSCs) with regioregular poly(3-hexylthiophene) (P3HT) as the donor and indene-C-60 bisadduct (ICBA) or indene-[6,6]-phenyl-C-61-butyric acid methyl ester (IPCBM) as the acceptor in the active layer and with Al or Ca/Al as the cathode. A remarkable improvement in the power conversion efficiency (PCE) (measured in air) was observed upon insertion of this ET layer, which increased the PCE from 5.78 to 7.5% for a PSC with ICBA and Ca/Al (5.53 to 6.63% with IPCBM) and from 3.87 to 6.88% for a PSC with ICBA and Al (3.06 to 6.21% with IPCBM). This ET layer provides multiple functionalities: (1) it generates an optical interference effect for redistribution of light intensity as an optical spacer; (2) it blocks electron-hole recombination at the interface with the cathode; (3) it forms an interfacial dipole that promotes the vacuum level of the cathode metal; and (4) it enhances electron conduction, as evidenced by (1) the increase in total absorption of 1:1 w/w P3HT:ICBA by a factor of 1.3; (2) the reduction in the hole-only current density profile by a factor of 3.3 at 2.0 x 10(5) V/cm; (3) the decrease of 0.81 eV in the work function of Al from 4.28 to 3.47 eV, as determined by UV photoelectron spectroscopy; and (4) the decrease in the series resistance of PSCs with ICBA and Al by a factor of 4.5, as determined by the current-voltage characteristic under dark conditions; respectively. The PSC of 7.5% is the highest among the reported values for PSC systems with the simplest donor polymer, P3HT.