As organic photovoltaic effi ciencies steadily improve, understanding degradation pathways becomes increasingly important. In this paper, the stability under prolonged illumination of a prototypical polymer:fullerene active layer is studied without the complications introduced by additional layers and interfaces in complete devices. Combining contactless photoconductivity with spectroscopy, structural characterization at the molecular and fi lm level, and quantum chemical calculations, the mechanism of photoinduced degradation in bulk heterojunctions of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is studied. Bare fi lms are subjected to four conditions for 1000 h with either constant illumination or dark and either ambient or inert atmosphere. All samples are found to be intrinsically stable for 1000 + h under inert conditions, in contrast to complete devices. While PCBM stabilizes P3HT fi lms exposed to air, its fullerene cage is found to undergo a series of oxidations that are responsible for the deterioration of the photoconductivity of the material. Quantum chemical calculations show that PCBM oxides have deeper LUMO levels than pristine PCBM and therefore act as traps for electrons in the PCBM domains.

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As a way to control the surface properties of nanowires and nanotubes, we present a method for growing polymer from the surface of silicon/silica core/shell nanowires. After modification of nanowire surfaces with polymer initiators, Atom Transfer Radical Polymerization (ATRP) was used to grow methacrylate polymer chains from the surface. The resulting structures were characterized by SEM, TEM, and EELS. After etching the silicon cores, the resulting polymer-coated nanotubes will have hydrophilic silica cores with hydrophobic polymer shells.

We have prepared two series of first-generation thiophene-bridge dendrimers, with either three (3G1) or four (4G1) arms attached to a phenyl core, to elucidate their structure−property relationships. Optical properties were investigated with a combination of steady-state and time-resolved spectroscopic techniques. Steady-state spectroscopic data for the 3-arm dendrimers suggests that the exciton is delocalized over the α-conjugated thiophene segment and the phenyl core, but that the meta-linking of the dendrons prevents their electronic communication. In contrast, conjugation through the core to dendrons in the ortho and para positions is permitted in the 4-arm dendrimers, although the data suggest that the conjugation length does not extend over the full length of the α-conjugated sections of two coupled dendrons. This observation is due to steric interactions between neighboring arms, which forces the arms to twist and bend out of the plane of the phenyl core, and is particularly prevalent in disrupting the conjugation through the ortho positions. As expected, our results show that an increase in the bridge length results in an increase in the conjugation length for both dendrimers, and a subsequent red-shift of the absorption and emission. In addition, an increase in the dendron length results in an increase in the photoluminescence quantum yield and lifetime, suggesting that the ground and excited-state geometries are very similar and that the electronic transition is coupled to fewer vibrational modes.

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The combination of electron-rich and electron-poor moieties in conjugated molecules is frequently utilized in order to red shift the absorption spectrum and improve photon harvesting in bulk heterojunction photovoltaic devices. In this study we characterize a conjugated thiophene dendrimer that has an electron-withdrawing core and electron-rich dendrons in order to investigate the effects of this design approach on the salient properties that influence the performance of photovoltaic devices with this dendrimer donor. Beside the absorption onset, these properties are the morphology of dendrimer:fullerene films and the dynamics of photoinduced carrier generation and loss. For comparison we also characterize a control dendrimer with the same structure but without the electron-withdrawing core. In addition to lowering the band gap by ca. 0.5 eV, the electron-withdrawing core also planarizes the dendrimer resulting in enhanced order in bulk heterojunction films. We observe longer photocarrier lifetimes in this ordered structure compared to the films of the predominantly amorphous control. The characterization of dendrimer:fullerene bulk heterojunction photovoltaic devices shows no voltage loss despite the decreased absorption onset. The properties of the device are consistent with the improved photocarrier lifetimes, but they are limited by a low short-circuit photocurrent density. We attribute this to electron confinement in the core that hinders transfer to the fullerene acceptor.