Understanding the Structural Evolution of Single Conjugated Polymer Chain Conformers.

Single molecule photoluminescence (PL) spectroscopy of conjugated polymers has shed new light on the complex structure⁻function relationships of these materials. Although extensive work has been carried out using polarization and excitation intensity modulated experiments to elucidate conformation-dependent photophysics, surprisingly little attention has been given to information contained in the PL spectral line shapes. We investigate single molecule PL spectra of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) which exists in at least two emissive conformers and can only be observed at dilute levels.

Spectroscopic and Intensity Modulated Photocurrent Imaging of Polymer/Fullerene Solar Cells.

Molecular spectroscopic and intensity modulated photocurrent spectroscopy (IMPS) imaging techniques are used to map morphology-dependent charge recombination in organic polymer/fullerene solar cells. IMPS uses a small (∼10%) sinusoidal modulation of an excitation light source and photocurrent responses are measured while modulation frequencies are swept over several decades (∼1 Hz-20 kHz). Solar cells consisting of either poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) blended with a soluble fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are used as targets.

Azulene methacrylate polymers: synthesis, electronic properties, and solar cell fabrication.

We report the synthesis of novel azulene-substituted methacrylate polymers by free radical polymerization, in which the azulene moieties represent hydrophobic dipoles strung pendant to the polymer backbone and impart unique electronic properties to the polymers. Tunable optoelectronic properties were realized by adjusting the azulene density, ranging from homopolymers (having one azulene group per repeat unit) to copolymers in which the azulene density was diluted with other pendant groups. Treating these polymers with organic acids revealed optical and excitonic behavior that depended critically on the azulene density along the polymer chain.

Morphology-dependent electronic properties in cross-linked (P3HT-b-P3MT) block copolymer nanostructures.

Combined Kelvin probe force microscopy and wavelength-resolved photoluminescence measurements on individual pre- and post-cross-linked poly(3-hexylthiophene)-b-poly(3-methyl alcohol thiophene) (P3HT-b-P3MT) nanofibers have revealed striking differences in their optical and electronic properties driven by structural perturbation of the crystalline aggregate nanofiber structures after cross-linking. Chemical cross-linking from diblock copolymer P3HT-b-P3MT using a hexamethylene diisocyanate cross-linker produces a variety of morphologies including very small nanowires, nanofiber bundles, nanoribbons, and sheets, whose relative abundance can be controlled by reaction time and cross-linker concentration. While the different cross-linked morphologies have almost identical photophysical characteristics, KPFM measurements show that the surface potential contrast, related to the work function of the sample, depends sensitively on nanostructure morphology related to chain-packing disorder.

Cross-linked functionalized poly(3-hexylthiophene) nanofibers with tunable excitonic coupling.

We show that mechanically and chemically robust functionalized poly(3-hexylthiophene) (P3HT) nanofibers can be made via chemical cross-linking. Dramatically different photophysical properties are observed depending on the choice of functionalizing moiety and cross-linking strategy. Starting with two different nanofiber families formed from (a) P3HT-b-P3MT or (b) P3HT-b-P3ST diblock copolymers, cross-linking to form robust nanowire structures was readily achieved by either a third-party cross-linking agent (hexamethylene diisocyanate, HDI) which links methoxy side chains on the P3MT system, or direct disulfide cross-link for the P3ST system.

Packing dependent electronic coupling in single poly(3-hexylthiophene) H- and J-aggregate nanofibers.

Nanofibers (NFs) of the prototype conjugated polymer, poly(3-hexylthiophene) (P3HT), displaying H- and J-aggregate character are studied using temperature- and pressure-dependent photoluminescence (PL) spectroscopy. Single J-aggregate NF spectra show a decrease of the 0-0/0-1 vibronic intensity ratio from ~2.0 at 300 K to ~1.

Resonance Raman studies of excited state structural displacements of conjugated polymers in donor/acceptor charge transfer complexes.

Resonance Raman spectra of poly(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and small molecule acceptor blend charge transfer (CT) complexes reveal long and detailed progressions of overtone and combination bands. These features are sensitive to the specific MDMO-PPV/acceptor interactions and enable quantitative calculations of vibrational mode specific displacements of the polymer CT complex.

Effect of fullerene intercalation on the conformation and packing of poly-(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene).

Photoluminescence (PL) and resonance Raman spectroscopy are used to track changes in the conformations and packing of poly-(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) chains with the addition of [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. PL lineshapes of MDMO-PPV thin films as a function of annealing time were first measured to determine the spectroscopic signatures of chain conformations and packing in the absence of PCBM. Annealing results in enhanced interchain interactions leading to red-shifts of PL 0-0 transitions by up to ∼300 cm(-1) and apparent increases of the line shape Huang-Rhys factors.

Observation of the missing mode effect in a poly-phenylenevinylene derivative: effect of solvent, chain packing, and composition.

Optical emission spectra of poly[2-methoxy-5-[3('),7(')-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (∼1225 cm(-1)) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600 cm(-1) contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schrödinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities.