No abstract

We report on the fabrication of photovoltaic cells, PVs, with controlled donor/acceptor interfaces using a process based on the phase separation between a cross-linkable polyfluorene and polystyrene. Robust, nanostructured columnar-grain layers of a conjugated cross-linked polymer, F8T2Ox1 (an oxetane-functionalized derivative of poly(9,9-dioctylfluorene-alt-bithiophene)) are obtained after removal of polystyrene. These layers are used, in combination with 1-(3-methoxycarbonyl)propyl-1phenyl-(6,6)C 61 (PCBM) deposited by spin coating, to define donor/acceptor interfaces, as PVs' active layers. The performance of these cells is dependent on the dimensions of the surface structures. In particular, a significant power conversion efficiency improvement is observed upon decrease of column diameter, reflecting an improvement of the exciton dissociation. We find, however, that these efficiencies still fall below those of the PVs based on blends of the same components, but are larger than the ones found for planar bilayer PVs. Furthermore, PVs based on blends of cross-linked F8T2Ox1 and PCBM showed enhanced efficiency and thermal stability with respect to PVs based on blends of PCBM and the non-cross-linkable analogue poly(9,9-dioctylfluorene-alt-bithiophene). Taking into account that the columnar-grain morphology is recognised as the ''ideal'' architecture for PVs' active layer provided the column radii are of the order of few nanometres, this work gives a new insight into how to achieve efficient organic photovoltaic cells through the use of cross-linkable conjugated polymers as the electron-donor component.

We show that molecular wires up to 14 nm in length composed by zinc-porphyrins bridged by bipyridines stand efficient electrical transport. Self-assembled molecular wires were prepared step-by-step, alternating up to 13 units of zinc-octaethylporphyrin with axially coordinated 4,4′-bipyridine, on highly oriented pyrolytic graphite (HOPG). A combination of molecular resolution imaging and scanning tunneling spectroscopy allowed us to follow molecules self-assembly in real time during wire fabrication and to measure wires current, respectively. A statistical analysis of hundreds of current–voltage curves was carried out to determine the conductance of individual porphyrin/bipyridine wires. From the conductance dependence on the wires length an ultra low attenuation factor (β = 0.015 ± 0.006 Å–1) was obtained for shorter wires, with a transition in conduction regime occurring at ca. 6.5 nm long wires.

Optical memories with long-term stability at high temperatures have long been pursued in azopolymers with photoinduced birefringence. In this study, we show that the residual birefringence in layer-by-layer (LbL) films made with poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) alternated with poly(allylamine hydrochloride) (PAH) can be tuned by varying the extent of electrostatic interactions with film fabrication at different pHs for PAH. The dynamics of both writing and relaxation processes could be explained with a two-stage mechanism involving the orientation of the chromophores per se and the chain movement. Upon calculating the activation energies for these processes, we demonstrate semiquantitatively that reduced electrostatic interactions in films prepared at higher pH, for which PAH is less charged, are responsible for the longer stability at high temperatures. This is attributed to orientation of PAZO chromophores via cooperative aggregation, where the presence of counterions hindered relaxation.