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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.

Insoluble patterns of cross-linkable conjugated polymers, CPs, are obtained from spin coating their blends with polystyrene, PS, following an approach first demonstrated for poly [2-methoxy-5-(2 0 -ethyl)hexyloxy-2,4-phenylenevinylene] (MEH-PPV) by Castro et al. (Chem. Mater. 2006, 18, 5504). Taking advantage of the CP and PS tendency to phase separate, we make use of CP functionality to be crosslinked, so PS can be removed, leaving behind a solvent-resistant pattern. Columnar, spike, and porous structures can be obtained. Furthermore, we show that these patterns morphologies and dimensions can be controlled by adjusting the experimental conditions and the PS molecular weight. We present strategies to tune cross-linkable CP's films morphologies aiming for applications in organic solar cells and light-emitting devices.

Electrostatic interactions govern most properties of polyelectrolyte films, as in the photoinduced birefringence of azo-containing polymers. In this paper we report a systematic investigation of optical storage characteristics of cast and layer-by-layer (LbL) films of poly[1 -[4-(3-carboxy-4 hydroxyphenylazo) benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). Birefringence was photoinduced faster in PAZO cast films prepared at high pHs, with the characteristic writing times decreasing almost linearly with the pH in the range between 4 and 9. This was attributed to an increased free volume for the azochromophores with the enhanced electrostatic repulsion in PAZO charged to a greater extent. In contrast, in LbL films of PAZO alternated with poly(allylamine hydrochloride) (PAH), the electrostatic interactions between the oppositely charged polymers hampered photoisomerization and molecular rearrangement, thus leading to a slower writing kinetics for highly charged

Molecular-scale devices can be made using a step-by-step procedure, in a controllable and highly versatile way. In this report, we describe the growth of molecular wires (MW) from zinc (II) octaethylporphyrin (ZnOEP) assembled on highly oriented pyrolytic graphite (HOPG) by a step-by-step approach using 4,4'-bipyridine (BP) to bridge the porphyrin units, via coordination of the nitrogen atom to zinc. In order to gain an insight into the molecular self-organization of these wires, we carried out a detailed scanning tunnelling microscopy (STM) analysis of each monolayer, using a solid/liquid interface technique, up to a complete ZnOEP/BP/ZnOEP/BP/ZnOEP-assembled structure. The electrical properties of the MWs were assessed by scanning tunnelling spectroscopy (STS) and by current-sensing atomic force microscopy (CS-AFM), showing an increase of electrical resistance with the length of the MW.

Electrostatic and hydrophobic interactions govern most of the properties of supramolecular systems, which is the reason determining the degree of ionization of macromolecules has become crucial for many applications. In this paper, we show that high-resolution ultraviolet spectroscopy (VUV) can be used to determine the degree of ionization and its effect on the electronic excitation energies of layer-by-layer (LbL) films of poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). A full assignment of the VUV peaks of these polyelectrolytes in solution and in cast or LbL films could be made, with their pH dependence allowing us to determine the pK(a) using the Henderson-Hasselbach equation. The pK(a) for PAZO increased from ca. 6 in solution to ca. 7.3 in LbL films owing to the charge transfer from PAH. Significantly, even using solutions at a fixed pH for PAH, the amount adsorbed on the LbL films still varied with the pH of the PAZO solutions due to these molecular-level interactions. Therefore, the procedure based on a comparison of VUV spectra from solutions and films obtained under distinct conditions is useful to determine the degree of dissociation of macromolecules, in addition to permitting interrogation of interface effects in multilayer films.